1. Field of the Invention
The present invention relates to a driving circuit and a method of driving an active-matrix type organic electroluminescent device, and more particularly to a driving circuit and a method of driving an organic electroluminescent device having the capability of shortening a time required to display picture data on a screen, wherein the picture data applied from a current driver integrated circuit (IC) is of a minimum gray level.
2. Discussion of the Related Art
A related art organic electroluminescent device driving circuit will now be described with reference to the accompanying drawings.
FIG. 1 generally illustrates a block diagram of a driving circuit used in an active-matrix type organic electroluminescent device. The driving circuit includes a gate driver unit 20 for sequentially outputting a control signal to select gate lines in a luminescent array unit 10 and a current driver unit 30 for supplying picture data to data lines in the luminescent array unit 10 corresponding to gate lines that are selected by the gate driver unit 20 and selectively driving organic electroluminescent devices connected to the selected line.
FIG. 2 illustrates a driving circuit unit used in an organic electroluminescent device. The driving circuit unit includes first and second PMOS transistors PM1 and PM2, wherein the sources of the first and second PMOS transistors are connected to a power voltage (VDD) and wherein the gates of the first and second PMOS transistors are commonly connected; a first capacitor C1 connected between the power voltage (VDD) and the commonly connected gates of the first and second PMOS transistors PM1 and PM2; an organic electroluminescent device 11 connected between a drain of the first PMOS transistor PM1 and a ground (VSS); a source of a third PMOS transistor PM3 connected to the commonly connected gates of the first and second PMOS transistors; a drain of the third PMOS transistor PM3 connected to a drain of the second PMOS transistor PM2, so as to be energized as a gate of the third PMOS transistor receives a control signal from the gate driver unit 20; a source of a fourth PMOS transistor PM4 connected to commonly connected drains of the second and third PMOS transistors PM2 and PM3, so as to be energized as a gate of the fourth PMOS transistor receives a control signal of the gate driver unit 20; and a first NMOS transistor NM1 connected between a drain of the fourth PMOS transistor PM4 and the ground (VSS), so as to be energized as a gate of the first NMOS transistor NM1 receives an analog voltage, corresponding to the picture data from the current driver 30.
An operation of the electroluminescent device illustrated in FIGS. 1 and 2 will now be described.
When a line in the luminescent array unit 10 is selected by a control signal from the gate driver unit 20 shown in FIG. 1, a low potential signal is applied from the driving circuit unit in the organic electroluminescent device to the gates of the third and fourth PMO transistors PM3 and PM4, so that the third and fourth PMOS transistors PM3 and PM4 shown in FIG. 2 may be energized.
Analog voltages, corresponding to picture data, may be applied from the current driver unit shown in FIG. 1 to the gate of the first NMOS transistor NM1 shown in FIG. 2. In applying the analog voltages, the degree to which the first NMOS transistor NM1 is energized may be controlled.
A proper voltage value may therefore be outputted from the current driver unit 30 according to the gray level characteristics of each of the individual organic electroluminescent devices 11. For example, if a gray level is to be implemented as a 8 bit digital data signal, the current driver 30 converts digital values between a predetermined maximum gray level of, for example, ‘11111111’ and a predetermined minimum gray level of, for example, ‘00000000’ to analog voltage values using a digital/analog converter. The digital/analog converter applies the analog voltage values to gates of the first NMOS transistors NM1, thereby controlling the degree to which the first NMOS transistors NM1 are energized.
When the third and fourth PMOS transistors PM3 and PM4 are energized, a predetermined amount of current flows through a first route beginning at the power voltage (VDD) to the second and fourth PMOS transistors PM2 and PM4, from the second and fourth PMOS transistors to the first NMOS transistor NM1, and from the first NMOS transistor to ground (VSS). The predetermined amount of current flows through the first route according to the degree to which the first NMOS transistor NM1 is energized by the analog voltage value supplied from the current driver unit 30. According to the principles of current mirroring, a predetermined amount of current also flows through a second route beginning at the power voltage (VDD) then flowing to the first PMOS transistor PM1, then to the organic electroluminescent device 11, and lastly to ground (VSS) thereby controlling luminescent characteristics of the organic electroluminescent device 11.
If a predetermined maximum gray level is to be displayed by the organic electroluminescent device 11, the current driver unit 30 converts a digital value of, for example, ‘11111111’ into a corresponding analog voltage value and applies the corresponding analog voltage value to the gate of the first NMOS transistor NM1. Then, the degree to which the first NMOS transistor NM1 is energized, is maximized allowing a maximum amount of current to flow through the first route. Accordingly, a maximum amount of current also flows through the second route, so that the predetermined maximum gray level may be displayed by the organic electroluminescent device 11.
If a predetermined minimum gray level is to be displayed by the organic electroluminescent device 11, the current driver unit 30 converts a digital value of, for example, ‘00000000’ into a corresponding analog voltage value and applies the corresponding analog voltage to the gate of the first NMOS transistor NM1. Then, the first NMOS transistor NM1 is turned off, e.g., placed in a floating state, such that no current flows through either the first or second routes so that the predetermined minimum gray level may be displayed by the organic electroluminescent device 11.
The gate driver unit 20 sequentially outputs a series of control signals so that the first through the last gate lines in the luminescent array unit 10, in which a plurality of the organic electroluminescent devices 11 are arranged, may be sequentially selected to display one frame of a picture on a screen.
Assuming that the organic electroluminescent device 11 illustrated in FIG. 2 is coupled to the first gate line in the luminescent array unit 10, the third and fourth PMOS transistors PM3 and PM4 may be energized when the first line is selected by the gate driver unit 20. Accordingly, an analog voltage value specific to the organic electroluminescent device 11 may be applied to the gate of the first NMOS transistor NM1 by the current driver unit 30 to control the degree to which the first NMOS transistor NM1 is energized. Accordingly, a predetermined amount of current flows to the first and second routes so that a proper gray level may be displayed by the organic electroluminescent device 11.
After the first gate line has been selected by the gate driver unit 20, the next consecutive gate line is selected and the third and fourth PMOS transistors PM3 and PM4 coupled to the first gate line are turned off. Accordingly, the gray level of the corresponding organic electroluminescent device 11 on the first gate line is maintained by the first capacitor C1 until the last gate line in the luminescent array unit 10 is selected, thereby displaying one frame of a picture on a screen.
However, the related art driving circuit illustrated in FIGS. 1 and 2 has the following problem. When an organic electroluminescent device consecutively displays a maximum gray level in a first frame of a picture and then again in a second frame, the first NMOS transistor NM1 energized in the first picture frame turned off and induced into a floating state. The voltage charged in the first capacitor C1 is then gradually reduced from the maximum gray level to the minimum gray level. Accordingly, it is impossible to accurately display the appropriate gray level within an organic electroluminescent device. Further, it becomes difficult to drive the organic electroluminescent devices with a quick response speed.